A recent trend in the use of composite photography for motion pictures and television is the virtual studio that substitutes computer renderings in place of real sets.
A large curved blue backing permits actors to perform their scenes in front of this backing. The large blue backing permits a wide range of motion of the actors. As the camera follows the movements of an actor, any background scene in the computer must be moved in the opposite direction or the background scene will appear to be frozen to the image frame. Sensors on the camera dolly and camera provide the information needed by the computer to properly move the background scene image. Other methods use very small marks on the screen to provide camera moves to the computer.
Because of the similarity of the words “backing” and “background”, the term “blue screen” is used to identify the plain colored backing behind the subjects during photography. The term “background scene” is used to describe the images that replace the blue screen. The selection of the blue screen is typically chosen because this color is easy to avoid in ones wardrobe. However if bright or deep blue is required in the wardrobe, one simply switches to a green screen.
There are many times when the camera image frame overshoots the blue screen and exposes part of the interior of the stage. An example of this overshoot is when the actor is required to appear to be at quite a distance. The camera is moved back from the actor and blue screen, and the lens is set to a shorter focal length. The actor now appears to be far away and is quite small in the image frame.
The image frame, being larger than the blue screen, exposes part of the stage ceiling, side walls and floor, plus all the garbage consisting of electrical cables, junction boxes, lamp stands, chairs, portable dressing rooms, people and miscellaneous other items such as a garbage can. A compositing device, such as the Ultimatte 11 available from Ultimatte Corporation, or equivalent computer, generates garbage mattes. Each of these mattes may be brought in from the edge of the image frame up to the edge of the blue screen, thus covering up all the garbage. When mattes are brought up to all four edges of the blue screen area they create a window frame, framing the blue screen. This is likely the origin of the term window and garbage matte. The term ‘matte’ herein refers to the garbage matte.
Matte areas are assigned a signal level of 1.0 to turn on the background scene to full level in the image area covered by the mattes. The blue screen also generates a control signal, Ec, where Ec=B−f(G,R), and R, G and B are red, blue and green signal levels and f(G,R) is a function of G and R such as the maximum of G and R. Its purpose is to turn on the background scene to full level within the un-obscured blue screen area, and to reduce the signal level of the background scene in proportion to the subject's opacity. Ec becomes zero for opaque subjects. The Ec signal level generated from the blue screen luminance is rarely sufficiently uniform to match the fixed 1.0 signal level of garbage mattes. A mismatch between these two signal levels of as little as one percent, where the matte joins the blue screen, is clearly visible in most background scenes.
The visibility of this mismatch is substantially reduced by softening, or feathering the garbage matte and extending it a short distance into the blue screen area. A soft edge in the form of an “S” shaped ramp links the two levels. The disadvantage of this practice is the fact that the background is turned on by the garbage matte whether a subject is present or absent. When a subject is present in this soft edge area, the background is still on and makes the subject transparent. The need for an actor to avoid this transition area reduces the useable blue screen area.
The invention described below is a method for reducing or eliminating the visibility of an edge mismatch between the blue screen and an adjoining matte, without introducing subject transparency, or reducing usable screen area.